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  • Photooxygenation of the Helimers of (−)-Isocolchicine: Regio- andFacial Selectivity of the [4 + 2] Cycloaddition with Singlet Oxygenand Surprising Endoperoxide Transformations
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  • Photooxygenation of the helimeric mixture of (−)-(M,7S)/(P,7S)-isocolchicine (6) with the superdienophile singlet oxygen has been studied. Cycloaddition occurred with high regioselectivity atthe 7a,11-positions of the alkaloid and predominantly at the diene face anti to the amidic substituentat the stereogenic center C-7, leading to two endoperoxides 7 (syn) and 8 (anti) with an 1:7 ratio.The structure of the minor product 7 was established by X-ray analysis. Investigation of thetriethylamine induced transformation of the predominant endoperoxide 8 furnished a mixture oftwo isomers (M,7S)-10a/(M,7S)-10b in a 2:1 ratio possibly with constitutional interconversion andwith (M,7S)-9 as plausible intermediate. Treatment of this mixture with silica gel/ethyl acetate atambient temperature surprisingly led to an atropenantiomerically pure colchicinoid (M,7S)-12characterized by an eightmembered oxocine B-ring, the structure and absolute configuration ofwhich could be determined by X-ray analysis. For the unprecedented formation of the novelcolchicinoid (M,7S)-12 a plausible reaction pathway is suggested, involving a complete transfer ofthe (M) helical asymmetry of the intermediate (M)-11 into (S) asymmetry of the newly formedcarbon center of (M,7S)-12. Prerequisite for such a scenario is the configurational stability of theintermediate pseudobiaryl (M)-11, under the conditions employed, allowing to transmit the axialchirality onto the chiral center of the product (M,7S)-12.
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